The present embodiments relate to a medical imaging system for combined magnetic resonance imaging and irradiation of an examination subject, and to a method for determining the configuration of shim units with shim elements.
When a magnetic resonance tomography system is operated in combination with an integrated radiation source (e.g., with an X-ray source or a LINAC (e.g., linear particle accelerator; radiotherapy source), an adjustment of the radiation application angle relative to the patient is desired in most cases for clinical reasons. Either (a) the MR magnet rotates about a longitudinal axis of the MR axis or (b) the remaining components rotate about the static MR magnet. The integrated radiation source is to be able to assume different angulations relative to the examination subject.
In magnetic resonance imaging, a maximally homogeneous magnetic field B0 is to be provided. The field homogeneity is generally in the order of magnitude of a few ppm and lower. Technically, it is not possible to produce magnets with perfect field homogeneity, mainly due to constraints during manufacture, a large number of variables in the manufacturing process, and mechanical and electrical tolerances. In addition, surrounding structures influence the magnetic field and generate field distortions.
In magnet technology, the technique known as shimming is used to compensate for small inhomogeneities that are typically present in magnetic fields, a distinction being made between active and passive shimming. For passive shimming, magnetized material is generally arranged at specific points of the MR scanner during the installation of the magnet. For active shimming, specially manufactured energizable coils (similar to gradient coils) are used. The current flow therethrough may be varied as appropriate in order to fine-tune the homogeneity of the magnetic field.
As a result of the shimming, the field homogeneity within the volume to be visualized is improved in accordance with the desired quality. In passive shimming, ferromagnetic materials (e.g., iron or steel) are arranged in this case in a distributed manner in regular patterns at specific points along the inner bore of the magnet. A typical arrangement for cylindrical superconducting scanners contains between 12 and 24 carrier drawers, known as “trays”, that are distributed symmetrically around the circumference of the magnet. Each shim tray is accommodated along the z-axis of the scanner in a type of channel and contains compartments into which a desired number of ferromagnetic shim elements may be inserted.
Known MR imaging systems generally make use of passive and active shimming simultaneously. Active shimming is employed only to shield against low-order (e.g., first- and second-order) field distortion harmonics. Higher orders are suppressed by passive shimming. The advantage of active shimming lies in the ability to make dynamic adjustments to the currents flowing through the coils. This enables the shimming to be modified in order to match the particular examination subject. In present-day MR systems, high-speed automated shimming is routinely performed during the preparatory phase prior to the examination. The disadvantage of passive shimming lies in the fact that passive shimming is a static solution. Changing or replacing the magnet or changing the environment of the magnet typically causes the magnetic field to become more inhomogeneous. As a result, the shimming is also to be adjusted when the magnet rotates about the longitudinal axis of the magnet around an environment of the magnet or when the environment rotates around the static magnet.
In the article “From static to dynamic 1.5 T MRI-linac prototype: impact of gantry position related magnetic field variation on image fidelity,” by Sjoerd Crijns and Bas Raaymakers, Phys. Med. Biol. 59, pages 3241-3247, 2014, magnetic field variations of a particle therapy LINAC that rotates on a gantry around a fixed MR magnet are investigated. Field inhomogeneities are reduced in this case by selectively driving the coils for the active shimming in an angle-dependent manner. Only first-order active shims (e.g., linear gradient) are used, while higher-order inhomogeneities are not addressed.
In the article “Geometric distortion and shimming considerations in a rotating MR-linac design due to the influence of low-level external magnetic fields” by K. Wachowicz, T. Tadic and B. G. Fallone, Med. Phys. 39(5), pages 2659-2668, 2012, theoretical solutions for shimming in the case of a rotating MR-LINAC system are discussed. The described solution mentions a passive shim unit that is connected to the rotating magnet and an active shim unit for first- and second-order distortions. Various approaches are examined. For example, a passive shim unit that is optimized for a single angle of rotation or a passive shim unit that offers an average coverage for the entire angular range is provided.